(E)-2-methyl-2-buteneoic acid ((E)-2M2BA) (commonly known as tiglic acid) and (Z)-2-methyl-2-buteneoic acid ((Z)-2M2BA) (commonly known as angelic acid) are useful starting materials for preparing flavors and fragrances, and for preparing pharmaceutical intermediates.
(Z)-2M2BA has been prepared by the oxidation of the corresponding (Z)-alcohol with manganese dioxide, followed by the oxidation of the resulting aldehyde with sodium chlorite in the presence of hydrogen peroxide and sodium hydrogen phosphate (M. Zaidlewicz, Z. Walasek, PL 169502 B1 (1996)). R. E. Buckles and G. V. Mock reported the preparation of (E)-2M2BA in 53% yield by the action of 100% sulfuric acid on 2-hydroxy-2-methylbutyronitrile, followed by hydrolysis of the resulting (E)-2-methyl-2-butenamide (J. Org. Chem., 15:680–684 (1950)). The (E)-2M2BA produced by the Buckles-Mock process was further converted to (Z)-2M2BA in 33% yield by bromination of (E)-2M2BA to 2,3-dibromo-2-methylbutyric acid, debromination to (Z)-2-bromo-2-methylbutenoic acid using methanolic potassium hydroxide, followed by reduction of the bromo-acid using sodium amalgam.
(E)-2M2BA has been prepared in 89% yield by the carbonylation of 2-chloro-2-butene using catalytic quantities of cetyltrimethylammonium bromide and nickel cyanide (H. Alper et al., Tetrahedron Lett., 30:2615–2616 (1989)). Similarly, treatment of (E)- or (Z)-2-bromo-2-butene with carbon monoxide and octacarbonyldicolbalt in the presence of methyl iodide and calcium hydroxide in dioxane-water gave only (E)-2M2BA in 87–88% yield (M. Masahiro et al., J. Chem. Soc., Perkin Transactions 1 (1):73–76 (1989)). (E)-2M2BA has also been produced in 3% yield by UV irradiation of (Z)-2M2BA for 43 days, followed by separation of the (E)- and (Z)-isomers by a combination of fractional crystallizations and extractions (S. W. Pelletier and W. L. McLeish, J. Am. Chem. Soc., 74:6292–6293 (1952)).
Nitriles are readily converted to the corresponding carboxylic acids by a variety of chemical processes, but these processes typically require strongly acidic or basic reaction conditions, high reaction temperatures, and produce unwanted byproducts and/or large amounts of inorganic salts as unwanted waste. These chemical processes for nitrile hydrolysis are not known to result in the regioselective hydrolysis of mixtures of geometric isomers of nitrites. U.S. Pat. No. 5,041,646 describes a process where (E,Z)-2M2BN is reacted with sulfuric acid at elevated temperatures, followed by distillation of the product mixture to produce a mixture of 80.5% (Z)-2M2BA and 19.5% (E)-2M2BA; pure (Z)-2M2BA was prepared by fractional crystallization of the (E,Z)-2M2BA mixture. 2-Methyl-3-butenenitrile (2M3BN), a commercially-available by-product produced during the manufacture of adiponitrile by hydrocyanation of butadiene, is readily isomerized to a mixture of (E)-2M2BN and (Z)-2M2BN, but separating this mixture by distillation is difficult and expensive due to the similar boiling points and chemical properties of the geometric isomers.
The enzyme-catalyzed hydrolysis of nitrile-containing substrates to the corresponding carboxylic acids is often preferred to chemical methods because the reactions are often run at ambient temperature, do not require the use of strongly acidic or basic reaction conditions, and produce the desired product with high selectivity at high conversion.
A nitrilase enzyme directly converts a nitrile to the corresponding carboxylic acid ammonium salt in aqueous solution without the intermediate formation of an amide. Nitrilases have been identified in a variety of microorganisms. For example, Kobayashi et al. (Tetrahedron 46:5587–5590 (1990); J. Bacteriology, 172:4807–4815 (1990)) have described an aliphatic nitrilase isolated from Rhodococcus rhodochrous K22 that catalyzed the hydrolysis of aliphatic nitrites to their corresponding carboxylic acid ammonium salts. A nitrilase from Rhodococcus rhodochrous NCIMB 40757 or NCIMB 40833 has been used to convert acrylonitrile to ammonium acrylate (U.S. Pat. No. 5,998,180). A nitrilase from Comamonas testosteroni has been isolated that can convert a range of aliphatic α,ω-dinitriles to either the corresponding ω-cyanocarboxylic acid ammonium salt or dicarboxylic acid diammonium salt (CA 2,103,616; S. Lévy-Schil et al., Gene, 161:15–20 (1995)). The regioselective hydrolysis of aliphatic α,ω-dinitriles to the corresponding ω-cyanocarboxylic acid ammonium salts by the nitrilase activity of Acidovorax facilis 72W has also been reported (Gavagan et al., J. Org. Chem., 63:4792–4801 (1998)). The nitrilase gene from Arthrobacter sp. NSSC104 has been cloned (WO 0314355), and a variety of bacterial nitrilases exhibiting stereoselectivity have been identified (WO 0300840).
A combination of two enzymes, nitrile hydratase and amidase, can also be used to convert aliphatic nitriles to the corresponding carboxylic acid ammonium salts in aqueous solution. Here the aliphatic nitrile is initially converted to an amide by the nitrile hydratase and then the amide is subsequently converted by the amidase to the corresponding carboxylic acid ammonium salt. Bacterial genera (including Rhodococcus, Pseudomonas, Alcaligenes, Arthrobacter, Bacillus, Bacteridium, Brevibacterium, Corynebacterium, and Micrococcus) are known to possess a broad spectrum of various nitrile hydratase and amidase activities. Cowan et al. (Extremophiles, 2:207–216 (1998)) and Martinkova and Kren (Biocatalysis and Biotransformation, 20:73–93 (2002)) have reviewed the nitrilase and nitrile hydratase/amidase enzyme systems of nitrile-degrading microorganisms.
Effenberger and Osswald (Tetrahedron, 12:2581–2587 (2001)) have reported the (E)-selective hydrolysis of (E,Z)-α,β-unsaturated nitriles by the recombinant nitrilase AtNIT1 from Arabidopsis thaliana, where the (E)-isomer of the (E,Z)-3-substituted-acrylonitrile mixtures was exclusively hydrolyzed in the presence of the corresponding (Z)-isomer; no 2,3-disubstituted acrylonitriles were examined as substrate for this nitrilase. When (E,Z)-3-heptenenitrile was used as substrate with this same nitrilase, enrichment of one isomer was not observed for either nitrile or acid. Almatawah et al. (Extremophiles, 3:283–291 (1999)) have reported that the nitrilase of Bacillus pallidus Dac521 showed no activity for hydrolysis of cis-2-pentenenitrile, but was capable of hydrolyzing acrylonitrile, methacrylonitrile, or crotononitrile; no 2,3-disubstituted acrylonitriles were examined as substrate for this nitrilase. Zhao and Wang (Chinese J. Chem., 20:1291–1299 (2002)) demonstrated that the combined nitrile hydratase and amidase activities of Rhodococcus sp. AJ270 could be used for the enantioselective biotransformation of racemic p-substituted-α-methylenepropionitriles. Where the amidase was shown by Zhao and Wang supra to discriminate between the two amide hydration products produced by the nitrile hydratase, no 2,3-disubstituted acrylonitriles were examined as substrate for this combination of nitrile hydratase/amidase enzymes.
The problem to be solved, therefore, is the lack of a process for the facile preparation of (E)- and (Z)-2M2BA from a mixture of (E,Z)-2M2BN using an enzyme catalyst having either nitrilase activity or a combination of nitrile hydratase and amidase activities, where the enzyme catalyst is regioselective for the hydrolysis or hydration of one of the two geometric isomers of (E,Z)-2M2BN.